A tabular silver halide grain (hereinafter referred to as a “tabular grain”) has the following photographic properties:
1) the ratio of surface area to volume (hereinafter referred to as a “specific surface area”) is large and a large amount of sensitizing dye can be adsorbed to the surface, so that the color sensitization sensitivity is relatively high as compared with the intrinsic sensitivity;
2) when an emulsion containing tabular grains is coated and dried, the grains are oriented in parallel, so that the coated layer can be reduced in the thickness and the photographic light-sensitive material obtained can have good sharpness;
3) in an X-ray photographic system, when a sensitizing dye is added to the tabular grain, the silver halide cross-over light can be extremely reduced and therefore, the deterioration of image quality can be prevented;
4) light scattering is reduced and therefore, an image can be obtained with high resolution;
5) the sensitivity to blue light is low and therefore, when tabular grains are used in a green-sensitive layer or a red-sensitive layer, a yellow filter can be removed from the emulsion.
By virtue of these advantageous properties, tabular grains have been heretofore used in commercially available light-sensitive materials.
JP-B-6-44132 (the term “JP-B” as used herein means an “examined Japanese patent publication”) and JP-B-5-16015 disclose a tabular grain emulsion having an aspect ratio of 8 or more. The aspect ratio as used herein means a ratio of the diameter to the thickness of a tabular grain. The diameter of a grain as used herein means the diameter of a circle having an area equal to the projected area of a grain when the emulsion is observed through a microscope or an electron microscope. The thickness is shown by the distance between two parallel main surfaces constituting a tabular silver halide.
JP-B-4-36374 discloses a color photographic light-sensitive material which is improved in the sharpness, sensitivity and graininess by using tabular grains having a thickness of less than 0.3 μm and a diameter of 0.6 μm or more in at least one layer of green-sensitive emulsion layer and red-sensitive emulsion layer.
In recent years, with the progress of silver halide light-sensitive materials designed to higher sensitivity and smaller format, a color light-sensitive material having higher sensitivity and improved image quality is keenly demanded. To meet with this requirement, the silver halide emulsion is demanded to have higher sensitivity and more excellent graininess. Conventional tabular silver halide emulsions cannot cope with these requirements and more improvement of the performance is being demanded.
As the aspect ratio of a tabular grain is larger, the specific surface area is larger and the above-described advantageous properties of a tabular grain can be more effectively used. In other words, a larger number of sensitizing dyes are adsorbed to a larger surface area and a larger amount of light is absorbed per one grain, whereby higher sensitivity can be obtained. Therefore, many studies have been heretofore made to prepare tabular grains reduced in the thickness. JP-B-5-12696 discloses a method of oxidizing and thereby ineffectuating a methionine group in gelatin and preparing thin tabular grains using the gelatin as a dispersion medium, JP-A-8-82883 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method of ineffectuating the amino group and the methionine group in gelatin and preparing thin tabular grains using the gelatin as a dispersion medium, and JP-A-10-148897 discloses method of chemically modifying the amino group in gelatin to introduce at least two or more carboxyl groups and preparing thin tabular grains using the gelatin as a dispersion medium.
Tabular grains are preferably prepared by a method of performing the nucleation and/or growth by adding silver halide fine grains to the reactor holding an aqueous solution of protective colloid in place of adding an aqueous silver salt solution and an aqueous halide solution. The techniques on this method is disclosed in U.S. Pat. No. 4,879,208, JP-A-1-183644, JP-A-2-44335, JP-A-2-43535 and JP-A-2-68538. Also, a method for producing an ultrathin tabular grain emulsion having an average thickness of less than 0.07 μm is disclosed in EP-A-507701 and JP-A-10-239787. According to this production method, a mixing vessel is provided outside a reactor for performing the nucleation and/or grain growth of silver halide grains, an aqueous silver salt solution and an aqueous halide solution are fed to and mixed in the mixing vessel to form silver halide fine grains, and the formed fine grains are immediately fed to the reactor to perform nucleation and/or grain growth in the reactor. The ultrafine grains produced in the mixing vessel are, after the introduction into the reactor, scattered within the reactor by stirring and due to the fine grain size, the grains readily dissolve to release silver ion and halide ion and thereby cause uniform nucleation and/or growth. That is, tabular grains having high uniformity and a small thickness can be produced. In the above-described patent publications, examples describing the preparation of tabular grains are set forth, where the average tabular thickness is at least 0.042 μm and the aspect ratio is at most 40.
JP-A-7-230133, JP-A-8-87087 and JP-A-8-87088 disclose a method of producing an ultrathin tabular grain emulsion having a tabular thickness of less than 0.07 μm and an AgBr content of 50 mol % or more. According to this production method, a growth-controlling agent (triaminopyrimidine, 5,7-diiodo-8-hydroquinoline or a phenol having two iodine substituents) is added at the growth of tabular grains and thereby the tabular grains are grown while keeping the small thickness.
JP-A-10-104769 discloses a method for producing an ultrathin tabular grain emulsion having a tabular thickness of 0.01 to 0.3 μm and an AgBr content of 60 mol % or more. According to this production method, a growth-controlling agent (a compound containing one or more heterocyclic nitrogen quaternary base compound within one molecule) is added before the formation of tabular grains and thereby tabular grains having a small thickness are formed.
The technique for obtaining a high aspect ratio and the use of a growth-controlling agent are accompanied by a serious problem in that the coefficient of variation in the tabular thickness increases due to coalescence of tabular grains with each other. The coalescence means a phenomenon such that two or more tabular grains gather to form a secondary particle. When the coalescence takes place, reduction in the photographic performance is caused, specifically, deterioration of graininess, reduction in the concentration after development, increase in fogging and softening of photographic properties. The coalescence is a phenomenon of causing cohesion between main surfaces of tabular grains and this occurs more readily as the aspect ratio is higher and the amount of the growth-controlling agent adsorbed is larger, namely, the coverage of the adsorbed growth-controlling agent on the grain surface is higher.